Date: Fri, 26 Aug 2011 12:38:03 +0300
From: Anestis Bechtsoudis <bechtsoudis.a@...il.com>
To: full-disclosure@...ts.grok.org.uk
Subject: Re: Advisory: Range header DoS vulnerability
 Apache HTTPD 1.3/2.x (CVE-2011-3192)

On 08/24/2011 07:55 PM, Dirk-Willem van Gulik wrote:
>           Apache HTTPD Security ADVISORY
>           ==============================
>                     UPDATE 1
> 
> Title:       Range header DoS vulnerability Apache HTTPD 1.3/2.x
> 
> CVE:         CVE-2011-3192
> Last Change: 20110824 1800Z
> Date:        20110824 1600Z
> Product:     Apache HTTPD Web Server
> Versions:    Apache 1.3 all versions, Apache 2 all versions
> 
> Description:
> ============
> 
> A denial of service vulnerability has been found in the way the multiple 
> overlapping ranges are handled by the Apache HTTPD server:
> 
>      http://seclists.org/fulldisclosure/2011/Aug/175 
> 
> An attack tool is circulating in the wild. Active use of this tools has 
> been observed.
> 
> The attack can be done remotely and with a modest number of requests can 
> cause very significant memory and CPU usage on the server. 
> 
> The default Apache HTTPD installation is vulnerable.
> 
> There is currently no patch/new version of Apache HTTPD which fixes this 
> vulnerability. This advisory will be updated when a long term fix 
> is available. 
> 
> A full fix is expected in the next 48 hours. 
> 
> Mitigation:
> ============
> 
> There are several immediate options to mitigate this issue until a full fix 
> is available:
> 
> 1) Use SetEnvIf or mod_rewrite to detect a large number of ranges and then
>    either ignore the Range: header or reject the request.
> 
>    Option 1: (Apache 2.0 and 2.2)
> 
>           # Drop the Range header when more than 5 ranges.
>           # CVE-2011-3192
>           SetEnvIf Range (,.*?){5,} bad-range=1
>           RequestHeader unset Range env=bad-range
> 
>           # optional logging.
>           CustomLog logs/range-CVE-2011-3192.log common env=bad-range
> 
>    Option 2: (Also for Apache 1.3)
> 
>           # Reject request when more than 5 ranges in the Range: header.
>           # CVE-2011-3192
>           #
>           RewriteEngine on
>           RewriteCond %{HTTP:range} !(^bytes=[^,]+(,[^,]+){0,4}$|^$)
>           RewriteRule .* - [F]
> 
>    The number 5 is arbitrary. Several 10's should not be an issue and may be
>    required for sites which for example serve PDFs to very high end eReaders
>    or use things such complex http based video streaming.
> 
> 2) Limit the size of the request field to a few hundred bytes. Note that while 
>    this keeps the offending Range header short - it may break other headers; 
>    such as sizeable cookies or security fields. 
> 
>           LimitRequestFieldSize 200
> 
>    Note that as the attack evolves in the field you are likely to have
>    to further limit this and/or impose other LimitRequestFields limits.
> 
>    See: http://httpd.apache.org/docs/2.2/mod/core.html#limitrequestfieldsize
> 
> 3) Use mod_headers to completely dis-allow the use of Range headers:
> 
>           RequestHeader unset Range 
> 
>    Note that this may break certain clients - such as those used for
>    e-Readers and progressive/http-streaming video.
> 
> 4) Deploy a Range header count module as a temporary stopgap measure:
> 
>      http://people.apache.org/~dirkx/mod_rangecnt.c
> 
>    Precompiled binaries for some platforms are available at:
> 
> 	http://people.apache.org/~dirkx/BINARIES.txt
> 
> 5) Apply any of the current patches under discussion - such as:
> 
>    http://mail-archives.apache.org/mod_mbox/httpd-dev/201108.mbox/%3cCAAPSnn2PO-d-C4nQt_TES2RRWiZr7urefhTKPWBC1b+K1Dqc7g@mail.gmail.com%3e
> 
> OS and Vendor specific information
> ==================================
> 
> Red Hat: 	Option 1 cannot be used on Red Hat Enterprise Linux 4.
> 		https://bugzilla.redhat.com/show_bug.cgi?id=732928
> 
> NetWare:	Pre compiled binaries available.
> 
> Actions:
> ========
> 
> Apache HTTPD users who are concerned about a DoS attack against their server 
> should consider implementing any of the above mitigations immediately. 
> 
> When using a third party attack tool to verify vulnerability - know that most 
> of the versions in the wild currently check for the presence of mod_deflate; 
> and will (mis)report that your server is not vulnerable if this module is not 
> present. This vulnerability is not dependent on presence or absence of 
> that module.
> 
> Planning:
> =========
> 
> This advisory will be updated when new information, a patch or a new release 
> is available. A patch or new apache release for Apache 2.0 and 2.2 is expected 
> in the next 48 hours. Note that, while popular, Apache 1.3 is deprecated.
> 
> 
> _______________________________________________
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Everyone must be also aware of the "Request-Range" except the "Range"
field in the header.

>>From the byterange source
(http://svn.apache.org/repos/asf/httpd/httpd/branches/2.2.x/modules/http/byterange_filter.c)



    if (!(range = apr_table_get(r->headers_in, "Range"))) {
        range = apr_table_get(r->headers_in, "Request-Range");
    }


Advisories must take into account this case too.


Credits to Gappy.

-- 
===============================================
* Anestis Bechtsoudis                         *
* Undergraduate Student                       *
*                                             *
* Network Operation Center (NOC Group)        *
* Dept. of Computer Engineering & Informatics *
* University of Patras, Greece                *
*                                             *
* Email: bechtsoudis.a@...il.com              *
===============================================

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